A number of studies are underway which are designed to improve our understanding of the dynamics of the Ames Salmonella/microsome test and to make improvements in the standard protocols to make the test easier to run and/or to make the test results more readily interpretable. These studies include the effects of substances which may be present in environmental mixtures which interfere with a positive mutagenic response in the apparent absence of cellular toxicity; characterization of the new tester strains, TA97 and TA102; the effect of prolonged storage of mutagen solutions used for positive controls; and the kinetics of cell growth on the plate as function of the number of cells plated, histidine concentration or mutagen concentration.